The present invention relates to a process and apparatus for treating textile fabrics, and more particularly, to a process and apparatus for drying, shrinking and finishing fabrics wherein the fabric is moved forward with sufficient overfeed between two parallel running conveyor belts, and is treated alternately from above and below by air from blowing nozzles.
Such a process and apparatus are known, for instance, from either DE-A No. 23 19 464 or DE-A No. 30 23 225. In these known processes, the fabric web is guided between two conveyor belts and heated air is blown on the web alternately at high speed from above and below the web. The air passing the fabric surface removes humidity therefrom so that the fabric is dried. Mechanical movement and beating of the fabric to the conveyor belts loosens up the tension in the fibers and, in woven or knitted fabrics, also serves to shrink the fabric during drying.
While the shrinking efficiency of these well-known processes is rather good, the drying efficiency thereof is low. Since air always takes the path of least resistance, it only touches the surface of the fabric and thus only absorbs surface humidity. The humidity in the interior of the fabric is only removed when it has reached the fabric surface by diffusion or a capillary effect. Whether and how much of the blown air will pass through the fabric depends on the distance between the blowing nozzle and the fabric, as well as on air permeability which is an inverse function of residual humidity.
Another drier for textile fabrics is disclosed by U.S. Pat. No. 4,219,942. In this drier, air from inclined nozzles is blown upon the textile fabric lying flat upon a conveyor belt. Vibrations are imparted to the conveyor belt by rotating vibrators in order to allow the material to shrink. The drying performance of this drier is quite low, as is its shrinkage performance.
In a further drier, known from BE-A No. 752 312, conveyor belts are made to vibrate by rotating vibrators in order to allow the fabric to shrink.
EP-A No. 0 137 066 teaches a device for drying and finishing textile fabrics which gives conventional drying results together with outstanding shrinking results. These results are achieved by means of conveyor belts made of stainless steel or aramid material such as Kevlar.RTM.. The conveyors are provided with high frequency vibrations by means of beating rollers, and the vibrations are transferred to the fabric on the conveyor belts so that the tension in fibers and woven material can be loosened. With this type of drier, however, the resonant frequency of the vibrating conveyor belts is detuned by the fabric weight, which is a function of residual humidity.
At present, the machine known to have the best drying capacity is a suction drum, in which the fabric is compulsorily passed through in a direction from the outside toward the inside. In this machine, however, the fabric is fixed to the drum and cannot be lifted before the air stream has been cut and the change-over to the next drum or the machine exit has been released. Since there is no room for the fabric to move, it leaves the sieve drum with the same dimensions as when it entered the machine. Therefore, the shrinking potential inherent to the fabric is not reduced, and a special shrinking process must be added since fabric which will shrink 15 to 20% during the first washing is no longer commercially available.
A construction is known from FR-A No. 131 4360 which originally should eliminate the above-mentioned disadvantages of the sieve drum. However, this construction is unsuitable since it produces additional tensile stress in the fabric.
Finally, a device for tensionless drying of textile fabrics is known from DE-C No. 964 948. In this device, the fabric is moved forward on an upper conveyor belt as in a pendant loop drier, with the fabric loops lying on a lower conveyor belt, and the distance between the upper and lower conveyor belts is adjustable. The fabric is treated from above with low pressure air and from below with high pressure air.